spase://NASA/Catalog/ACE/Magnetic_Holes

Magnetic holes are marked decreases in the magnetic field intensity which possess pressure balance across the interfaces. This means that when the decrease in magnetic pressure is accounted for by a corresponding and equal increase in the thermal pressure of the background ions and electrons. The recognition of magnetic holes can be an imprecise process that is subject to the ambient conditions of the interplanetary magnetic field. Under highly disturbed conditions, a magnetic hole may not be clearly recognizable. They are most easily seen under conditions when the IMF is otherwise steady and displaying large amplitude.
Magnetic holes are routinely observed within the solar wind and have been observed in planetary magnetospheres. They are seen over a wide range of scale sizes, from signals lasting < 1 minute to those lasting over 1 hour. This list attempts to focus on those magnetic holes lasting over 15 minutes in the ACE dataset.
We characterize the magnetic holes listed here as having either rounded, square, or sawtoothed sides. This may be an indication that the spacecraft passed near the edge boundary of the hole or that the pressure balance is incomplete so that the hole is either expanding or contracting.